Adsorption, apparent preferential

It was apparent that the dense adsorption layer of HPC which was formed on the silica particles at the LCST plays a part in the preparation of new composite polymer latices, i.e. polystyrene latices with silica particles in the core. Figures 10 and 11 show the electron micrographs of the final silica-polystyrene composite which resulted from seeded emulsion polymerization using as seed bare silica particles, and HPC-coated silica particles,respectively. As may be seen from Fig.10, when the bare particles of silica were used in the seeded emulsion polymerization, there was no tendency for encapsulation of silica particles, and indeed new polymer particles were formed in the aqueous phase. On the other hand, encapsulation of the seed particles proceeded preferentially when the HPC-coated silica particles were used as the seed and fairly monodisperse composite latices including silica particles were generated. This indicated that the dense adsorption layer of HPC formed at the LCST plays a role as a binder between the silica surface and the styrene molecules. 

Typical of the sort of data needed to determine whether additives affect the interface is that provided by a study of the influence of n-heptyl compounds on the gel structure of dispersions containing polar solids in nonpolar vehicles (70). The influence of the polar heptyl compounds on the fluidity of dispersions of rutile and a fine silica (HiSil) in a dibasic ester, Plexol 201, is shown in Fig. 7. Apparently, the more polar rutile adsorbs all except the chloride and in these cases thinning results. HiSil has a lower F value and adsorbs only the amine and alcohol preferentially. Greases prepared from the least polar solid, Aerosil, are also least influenced by these additives (or even by more complex ones). Measurements of the solution isotherms for HiSil and Aerosil reveal significant adsorption of heptyl alcohol, but no detectable chloride adsorption in the same concentration range. 

The data of the electrophoretic behaviour of bubbles in aqueous electrolytes, obtained from Kelsey et a/. [185], Yon and Jordan [180] and Huddelston and Smith [181] are also in support to the isoelectric state found. The apparent negative charge on the bubbles in the surfactant-free electrolyte solution was ascribed to preferential adsorption of OH ions. 

The second exception was specific for HZSM-5 that had been acidified with ammonium chloride and which had large particle sizes. The differential heat curve at 416-423 K for these samples passed through a maximum at relatively low coverages. This behavior could be explained by the combination of three independent phenomena immobile adsorption, mass-transfer limitations, and preferential location of the most energetic acid sites in the internal pores of the zeolite structure. Apparently, the strongest sites were not accessible to ammonia when the first doses were introduced but became progressively covered when further ammonia was added. Electron paramagnetic resonance studies (93) provided data to support this hypothesis. 

Solutes are one of the major components of foods, and they have significant effects on their adsorption at fluid interfaces. In addition, the study of the effects of ethanol and/or sucrose on protein adsorption at fluid interfaces is of practical importance in the manufacture of food dispersions. The presence of ethanol in the bulk phase apparently introduces an energy barrier for the protein diffusion towards the interface. This could be attributable to competition with previously adsorbed ethanol molecules for the penetration of the protein into the interface. However, if ethanol causes denaturation and/or aggregation of the protein in the bulk phase, the diffusion of the protein towards the interface could be diminished. The causes of the higher rate of protein diffusion from aqueous solutions of sucrose, in comparison with that observed for water, must be different in aqueous ethanol solutions. Since protein molecules are preferentially hydrated in the presence of sucrose, it is possible that sucrose limits protein unfolding in the bulk phase and reduces protein-protein interactions in the bulk phase and at the interface. Both of these phenomena may increase the rate of protein diffusion towards the interface. Clearly, the kinetics of adsorption of proteins at interfaces are highly complex, especially in the presence of typical food solutes such as ethanol and sucrose in the aqueous phase. 

The isoelectric point was apparently unaffected at low silica coverages—below ca. 0.5 wt %. This fact indicates that silica adsorbs preferentially onto surface sites in a manner that does not affect average surface charge-pH behavior. A similar suggestion was made by Kononov et al. (35) in describing silica adsorption onto fluorite. 

Bartell and Sloan13 conducted adsorption experiments on mixtures of ethanol and ethyl carbonate. They found that ethanol is preferentially adsorbed when the original concentration of ethanol is less than that of the ethyl carbonate. When ethanol is present in greater concentrations, however, ethyl carbonate is preferentially adsorbed. Still further, they found that there is no apparent adsorption of either ingredient when the two are present in approximately equal molar concentrations. This is explained on the basis that at that point both ingredients are adsorbed to the same extent, leaving the concentration unchanged. 

Some interesting secondary solvent effects have been noted for adsorption of the polycyclic aromatic hydrocarbons and certain of their derivatives on alumina 14). The more nearly linear isomers (e.g., anthracene relative to phenanthrene, 2-bromonaphthalene relative to 1-bromonaph-thalene) are preferentially adsorbed from most solvents, owing to the apparent weak localization of these compounds on linear site complexes see Section 11-2B, There is also a tendency for the preferential adsorption of strong solvent molecules on these same linear site complexes, with the result that strong solvents (or their solutions in weaker solvents) behave as selectively stronger solvents toward the preferentially adsorbed linear aromatics, relative to less linear isomers. As a consequence the ratio of values for two such isomers varies sharply with the solvent used, despite the fact that Eq. (8-3) predicts that this ratio should remain constant for all solvents i.e., A. is generally constant for two or more isomers. Jn extreme cases the ratio of K" values for two isomers of this type can be varied by a factor of 10 or more, depending upon the solvent used (see Table 11-4). 

Caroline and co-workers have recently reported measurements of translational diffusion coefficients in solutions of PS in two mixed-solvent systems at or near theta conditions. In the solvent CCb-methanol (85), they observed the diffusion theta state, defined when the coefficient y of Equation 41 equals 0.5, to occur at 25°C and a volume fraction of CCI4, (fyCCU = 0.8025. In this system there is strong preferential adsorption of the polymer for CCI4, and it is not possible to define a true theta state such that y = a = V2 and A2 = 0 simultaneously. Under diffusion theta conditions, the concentration dependence of Dt apparently is closely described by the Pyun-Fixman hard-sphere model. In the mixed solvent benzene—2 propanol, polystyrene exhibits a true theta condition at T = 25.5°C and (benzene) = 0.04. Frost and Caroline confirmed that y = 0.5 within experimental error in this system (86) and report that values of the parameter fcf are scattered between the extreme values corresponding to the predictions of Yamakawa (and Imai) and the soft-sphere model of Pyun-Fixman (or the Freed theory). 

Amides are preferentially adsorbed from aqueous solutions. Maximum adsorption occurs close to the pzc. The controlling factor appears to be the squeezing-out effect of the water rather than specific interaction of the amide molecule with the electrode even though this apparently occurs according to the high work of adhesion of form-amide and DMF (Table 7.1.1). 

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